Conditional operator assignment with Nullable types?

The problem occurs because the conditional operator doesn’t look at how the value is used (assigned in this case) to determine the type of the expression — just the true/false values. In this case, you have a null and an Int32, and the type can not be determined (there are real reasons it can’t just assume Nullable<Int32>).

If you really want to use it in this way, you must cast one of the values to Nullable<Int32> yourself, so C# can resolve the type:

EmployeeNumber =
    string.IsNullOrEmpty(employeeNumberTextBox.Text)
    ? (int?)null
    : Convert.ToInt32(employeeNumberTextBox.Text),

or

EmployeeNumber =
    string.IsNullOrEmpty(employeeNumberTextBox.Text)
    ? null
    : (int?)Convert.ToInt32(employeeNumberTextBox.Text),